On Sat, Jun 2, 2012 at 3:59 AM, David Shi <davidgshi at yahoo.co.uk> wrote: > Can any one clarify what "while stack:" mean? It iterates as long as 'stack' has something that evaluates as true. My guess is that stack is a list, and the loop is removing elements from that list, so it'll keep going as long as there's anything in it. ChrisA